The invention is particularly applicable to the domain of multiple sub-carriers (hereinafter: multi-carrier) coded modulation, also known as OFDM (Orthogonal Frequency Division Multiplexing, namely). The basic principle of the OFDM modulation is to split the frequency band into N sub-bands, with N being typically a power of two and each sub-band transmitting data via a sub-carrier.
With reference to FIG. 1, the transmission multi-carrier modulation of a binary data BD first starts with the mapping of groups of bits (2 bits, 4 bits, . . . ) in complex numbers modulated in QAM of 2n bits, so as to obtain the N components X1, . . . XN frequency vector to be transmitted. This vector is then transformed from the frequency domain into time data using a frequency/time converter, for example an IFFT (Inverse Fast Fourier Transforms) and then multiplexed in an multiplexer MP. The data is then converted by a digital-to-analog converter, called D AC converter, sent on the line and received by a sequence of inverse processings—analog-to-digital converter ADC, demultiplexer DP, Fast Fourier Transform IFF—, so as to obtain the N components H1X1, . . . , HNXN frequency vector at the reception side.
This technique is used in many telecommunication standards and therefore makes it possible to split the useful band into N independent sub-bands.
An essential step in the digital data transmission is the conversion of digital signals into analog signals. For this purpose, the DAC converter operates at the sampling frequency of the digital signal, using a fixed number of bits for the representation of the input signal.
The input digital signal is then quantified and is thus subjected to truncation. To meet the constraints of the DAC, for example in the case of a 10-bit DAC, the digital signal is represented over exactly 10 bits before being transmitted. This quantification step creates a noise, called quantification noise, which is present from the moment of transmitting the signal. Therefore, this noise may limit the system performances sharply.
In addition, when the transmitted signal has power variations depending on the frequency, then the quantification noise affects the carrier frequencies of the transmitted signal in an SNR ratio (signal-to-noise ratio) different depending on the transmission frequency, which may result in a performance decrease. This effect results from the fact that all the available QAM (Quadrature and Amplitude Modulation) modulations (QPSK Quadrature Phase Shift Keying) to be attributed to the sub-carriers is limited: beyond a certain SNR ratio (for example 30 dB) an SNR loss always leads to a loss of performance, whereas an SNR ratio gain will not lead to a gain of performance when the modulation type offering the maximal rate (example QAM1024) is reached.
However, constraints are usually imposed on the transmission power which should not exceed a certain level. This maximal level may be set by a standard. Such a constraint is particularly present when transmitting on a carrier current, or PLC (Power Line Communication), a constraint imposing a limitation of power spectral density in the band of 0-30 MHz to a power of −50 dBm/Hz and to a power of −80 dBm/Hz in the band of 30-300 MHz.
In the example of this standard, the power of carriers which are higher than 30 MHz should therefore be reduced by 30 dB. To this end, a conventional method illustrated in FIG. 1 consists in pre-multiplying, prior to the application of the inverse transform IFFT, each complex component Xk by a coefficient Ak so as to be able to apply a variable power on the signal to be transmitted. In this example, the Ak coefficients are chosen so as all the carriers corresponding to a frequency higher than 30 MHz are accordingly reduced by 30 dB.
FIG. 2 illustrates the signals amplitude depending on the frequency in the spectral domain. Whatever the power spectral density of the transmitted signal is, quantification noise Qn is flat. Therefore, this quantification noise does not have the same shape as the transmitted effective signal (Es) which varies according to the spectral bands (low band: 0-30 MHz and high band above 30 MHz). As a result, the low band has an average SR signal-to-noise ratio (SNR) much higher than that of the high band. Thus, on the presented example, with a 10-bit converter, the high band has an SNR ratio (Rh) limited to 20 dB, which is unfavorable and strongly limits the throughput, while the Nr ratio of the low band is higher than 40 dB, which is quite acceptable. The limiting signal for the SNR ratio is the effective signal (Es), with the other signals (peak-to-average “PAR” and marginal “Marge”), parallel to the effective signal and significantly higher, have also higher SNR ratios.
A known solution for reducing the quantification noise in the high band (above 30 MHz) is to increase the converter bit number, because the more bits it has the weaker the quantification noise will be. For example, with a 15-bit converter, the SNR ratio of FIG. 2 becomes higher than 40 dB in all bands.
The disadvantage of this solution is the significantly high cost of converters with a large number of input bits.
Another solution is to use two converters, one for each band, which would result in obtaining an acceptable SNR in each band. The disadvantage of this method is also the high cost of converters.
To reduce the quantification noise, also known, from patent document FR 2730590, is a method for feedbacking the signal supplied at the quantification circuit input to reduce the quantification noise. The feedback signal is generated like a filtered difference between a sample of the N bits signal and a time matching sample of a quantified M bits signal, where M is lower than N. The feedback signal is subtracted from the input signal before the quantification, thereby introducing out-of-band noise in the input signal to reduce the band noise in the quantified signal.
This type of noise reducer operates through the adaptation of the signal before the digital-to-analog conversion so as to incorporate the conversion noise. The reducer is not very efficient because the implementation of the feedback signal is not easy.